Thermionic amplifier



ug. l2, 19M., B. B. .JAcoBsEN E1- AL 2,252,007' THERMIONIC AMPLIFIER Filed April 5, 1939 Fig. 1.

HT-r R2 RI c1 E z G a L C3 RS/ nge. M w

V' R5 c1 s C' a m C3 Z m L'rf w /pvears ddcaren /foC/fe Patented ug. 12, 1941 Bent Blow Jacobsen and Alleman Holly Roche,

' London, England, assignors to Interruittional-` Standard ElectrcCorporaton, New York, N. Y.

Application April 5, 1939, serial No. 266,128 In Great Britain April 2s, 193s e. 41 claims. (o1. 179-171) This invention relates to thermionic amplifiers of the kind in which a portion of the output is fed back to the input in negative phase and more particularly to suchl amplifiers in which the phase of the feedback voltage on the control grid of the first valve has been upset by the connection to the control grid of an external circuit of large impedance.

According to the invention this objection is overcome by the application of feedback of substantially perfect phase shift to an electrode of the valve other than the control grid. Where the feedback is to a tetrode or pentode valve the feedback may be applied to both the control grid and the screen grid. In the case of a pentode both the signal input and the feedback voltage may also be applied to the suppressor grid.

The invention will be fully understood from the following detailed description and the accompanying drawing in which Figs. 1 and 2 are schematic diagrams of two embodiments of the invention.

Fig. 1 shows the circuit of an amplifier in which the feedback voltage is conveyed to the screen grid of a tetrode (or pentode) via the large condenser C1. In this circuit the valve V1 has a voltage applied to its control grid from the generator G, its plate circuit being fed through the resistance R1. The resistance R2 is used to apply the required steady voltage to the screen grid. The bias for the control grid is obtained from the usual biasing resistance R3 shunted by the decoupling condenser C3. The load Z is fed through the condenser C2 and the feedback voltage is taken from the output bridge consisting of the resistances R5, Re, R7 and the plate cathode impedance of the valve. The resistance Rs is included in the grid circuit of the valve and is the resistance across which the feedback voltage is generated.

In a normal amplifier when the generator G has a high internal impedance the effective feedback voltage on the control grid will generally differ in magnitude and phase from the feedback voltage on the resistance Rs. This is due to the input capacity of the valve. rlhe phase shift is particularly objectionable since it tends to render the amplifier unstable. If, for instance, the generator impedance is Z1 ohms and the input impedance of the valve is Z2 ohms, then the ratio of feedback volts on the grid of the valve to the feedback voltage on Re is it i eing assumed that Ra is small compared with Z1 and Zz. Assume thatgthe mutual conductance for the control vgrid is mi and for the screen `grid is m2; if the effect of the feedback is'measured i at the anode of V1 .then if the feedbackfis apfis -again assumed to be small. If C1 is made large then aC'i will be negligible at all relevant frequencies, and the relative feedback effect in the anode circuit will be m2. mz is purely scalar and the phase angle of the feedback is therefore in this case not affected by the input capacity of the valve. In the rst case the same effect may be obtained by reducing Z1, the generator internal impedance, but this will be accompanied by decreasing gain and is therefore undesirable. In the second case there is no such decrease in gain.

In Fig. 1, however. feedback is applied to both the control grid and the screen grid, and the total feedback effect in the anode circuit is then the vectorial sum of the screen feedback and the control grid feedback. Since the screen grid feedback is of perfect phase shift the sum of the two feedbacks will be more correct in phase than would be the control feedback alone, the more so the larger the screen grid feedback. The reason for maintaining some feedback on the control grid is to increase the input impedance of the amplifier. Perfect phase shift means a phase shift of with respect to the signal voltage applied to the control grid.

If the lead from the condenser C1 .to the resistance Rs in Fig. 1 and also the lead from the generator G to the resistance Ra are made slider contacts lalong the resistance, it will then be obvious that the relationship between the feedback on the control grid and the screen grid may be varied in any required manner. Thus, if desired, the feedback on the screen grid may be made considerably greater than that on the control grid and this is the preferred condition, as under these circumstances the resultant phase shift of the feedback path will be more suitable.

Fig. 2 shows a further possible modification in which both feedback voltage and signal voltage are applied to the suppressor grid of a pentode aswell as to the control grid, feedback voltage being also applied to the screen grid. 'I'he designations of the circuit elements in Fig. 2 are exactly the same as in Fig. 1, and no further description is therefore necessary.

What is claimed is:

1. An amplifier system comprising' a` tube havf ing a cathode, an anode, a control grid and another grid, an input circuit ofl relatively high impedance 'connected between said control grid and said cathode, means for applying to said control grid through said input circuit a negative feedback voltage and means for applying to said other grid through a substantially negligible imv pedance a negative feedback voltage of substan-v tially 180 phase shift with respect to the signal voltage applied to said control grid, whereby the stability of said amplier is improved.

2, An amplifier according vto claim 1 wherein both of said feedback voltages are derived from the same source.

3. An amplifier system comprising a tube having a cathode, an anode, a control grid and a screen grid, an input circuit of relatively high impedance connected between said control grid and said cathode, a source of negative feedback voltage, means for applying through said input circuit a Voltage derived from said source to said control grid to cause a feedback at a phase shift of other than 180 with respect to the signal voltage applied to said control grid and means for supplying through a substantially negligible impedance a voltage derived from said source to said screen grid to cause a feedback at a phase shift of substantially 180 with respect to the signal voltage applied to said control grid.

4. An amplier according to claim 1, wherein said tube comprises a suppressor grid, further comprising means for connecting said input circuit to said suppressor grid, and means for applying a negative feedback voltage through said input circuit to said suppressor grid.

BENT BLOW JAcoBsEN. HOLLY ROCHE. 

